doc: add Terms of Service

R=r
CC=golang-dev
https://golang.org/cl/5913044

doc/tos.html

+<!--{
+	"Title": "Terms of service"
+}-->
+
+<p>
+The Go website (the "Website") is hosted by Google.
+By using and/or visiting the Website, you consent to be bound by Google's general
+<a href="http://www.google.com/intl/en/policies/terms/">Terms of Service</a>
+and Google's general
+<a href="http://www.google.com/intl/en/privacy/privacy-policy.html">Privacy Policy</a>.
+</p>

lib/godoc/godoc.html

@@ -51,9 +51,10 @@
 </div>
 
 <div id="footer">
-Build version {{html .Version}}.
+Build version {{html .Version}}.<br>
 Except as noted, this content is licensed under a
-Creative Commons Attribution 3.0 License.
+Creative Commons Attribution 3.0 License.<br>
+<a href="/doc/tos.html">Terms of Service</a>
 </div>
 
 </body>